1. The Field of the Invention
The present invention is related to methods and apparatus for forming a tubular prosthesis, and more specifically, to methods and apparatus for laser cutting a tubular fabric graft.
2. Description of the Related Art
Stents and vascular grafts of various designs are known for the treatment of aneurysms as well as for the treatment of occlusive diseases of the blood vessels or other ducts. A common type of tubular prosthesis includes a graft made of a biocompatible material having mechanical properties that can withstand varying internal pressures. The graft may be supported by an internal or external stent, or by a plurality of expandable circular wires. One such wire-supported intraluminal graft is disclosed in U.S. Pat. No. 5,782,904, issued Jul. 21, 1998.
Many grafts of the prior art, such as in the '904 patent, are made of porous textile material, usually a crimped or resiliently circular-knitted stocking of polymerized ethylene-glycol-terephthalate (Dacron.TM.). Such textile grafts must often be treated blood, or "pre-clotted," before they are implanted to improve initial leak-resistance and biocompatibility. In recent years, vascular grafts have been made of expanded polytetrafluoroethylene (PTFE) possessing a porosity and flexibility such that no pre-treatment with blood is necessary.
In general, tubular grafts and their respective support and/or attachment means fall into two major categories, self-expanding and pressure expandable. Self-expanding intraluminal tubular prostheses include grafts supported and/or attached via resilient or shape-memory material such as spring steel or Nitinol.TM.. Self-expanding material is capable of being formed in a configuration from which it may be compressed to a radially compact diameter for placement within a damaged vessel. At the time of use, the memory feature of these materials causes them to self-expand from the radially compact diameter to the expanded operative diameter.
Pressure-expandable tubular prostheses include grafts supported and/or attached via plastically deformable material such as stainless steel that is initially formed in its radially compact diameter. This type of material does not have memory, and will remain in the radially compact diameter until manually expanded. Typically, outwardly directed pressure is exerted upon the prosthesis through use of a balloon so as to cause radial expansion and resultant plastic deformation of the material to its operative diameter.
If individual circular wires are used as opposed to a cylindrical stent, consideration must be given to the attachment means between the wires and tubular graft such that uniform and durable support is provided. Some designs stitch the wires to the exterior of the tubular graft. This stitching may ultimately fail, however, and more importantly the support provided to the tube may be inadequate, especially when high negative pressures are present within the lumen of the tube. U.S. Pat. No. 5,782,904 describes the use of thin, stainless-steel undulating wires that are woven through the fabric of the tube such that spaced segments of each wire are outside the tube with the remainder of that wire inside the tube. In this manner, fairly even support is provided to withstand varying pressures in the lumen of the tube. One drawback, however, is the time-intensive nature of weaving a plurality of undulating wires in specific locations along the tube. The weave pattern must be laboriously pre-marked on the outside of the tube. The assembly process is made even more complex if the graft is branched, such as a bifurcated or so-called "trouser graft."
In the prior art processes for forming grafts, tubular lengths of fabric or PTFE material are cut to individual graft lengths using a heated cautery wire. Shears or other mechanical cutters are unsuitable for fabric grafts because the cut ends tend to fray. The use of a heated wire, however, creates difficulties such as fumes and excessive melting of the material, and is also fairly time-consuming and imprecise.
Lasers are often used for cutting textile material for garments and sailcloth, for example. Examples of the use of lasers in the textile industry can be seen in U.S. Pat. Nos. 4,588,871, 5,200,592, and 5,614,115. However, lasers have not been used for forming grafts, although they have been employed to perforate material for bioprosthetic applications. For instance, U.S. Pat. No. 5,326,356 discloses using a laser to render biocompatible material porous for use in skin grafts, and U.S. Pat. No. 4,729,766 discloses using a laser to form indentations in the exterior surface of an otherwise impermeable tube to encourage tissue ingrowth. In another example, U.S. Pat. No. 5,628,782 discloses the use of a laser to macroscopically perforate an outer tube for covering a fiber-wrapped vascular graft. In all these examples, the goal is to render an otherwise impermeable material porous.
Despite many advances in the field of tubular grafts, there remains a need for an improve method of forming such grafts, and in particular a need to shorten and automate the process for forming which will produce more uniform, and more efficacious, grafts.